Engine intake air charge dilution with recirculated engine exhaust gas is applied in automotive engine controls to inhibit production of the undesirable engine exhaust gas component of oxides of nitrogen (NOx). Precise control of intake air charge dilution can lead to significant NOx reduction with minimum engine performance deterioration. Dilution may be internal to the engine or may be external. In external dilution, engine exhaust gas is drawn from an exhaust gas conduit into the engine intake manifold through an "external" valved conduit. A degree of opening of a valve, such as a solenoid, within the conduit is varied to control dilution of the fresh air charge in the intake manifold.
Internal dilution is provided through variation in relative timing between intake and exhaust valve events of an engine cylinder to create a valve overlap period during which both valves are open allowing cylinder exhaust gas to backflow from the exhaust port into the chamber and perhaps even through the intake port to the cylinder intake runner or intake plenum. An exhaust camshaft phaser may be used to vary engine cylinder valve timing. Maximum internal dilution is provided when the exhaust camshaft phaser is in a full retard (or a minimum advance) position corresponding to a maximum valve overlap period. Precisely controlled internal dilution is known to yield a NOx reduction without a significant penalty in emissions of hydrocarbons (HC). Precision internal dilution control conventionally requires closed-loop camshaft phaser position control responsive to a feedback signal from a camshaft phaser position transducer indicating actual camshaft phaser position. A camshaft phaser drive command is varied in response to a difference between a desired camshaft phaser position and the position indicated by the feedback signal. Camshaft phaser position transducers are typically complex and expensive and may suffer reliability shortcomings.
Conventional dilution control approaches, whether internal or external, generally prescribe a degree of dilution of the engine fresh intake air charge as a function of engine operating conditions. A dilution control command that corresponds to the prescribed dilution is then referenced from a static calibration schedule or from a static calibrated function, and either an EGR valve or a camshaft phaser are driven in accord with the referenced command. The calibration conditions under which the static schedule or the static function was generated can change significantly over the life of the system, for example due to component aging, change in exhaust gas flow characteristic, for example due to system contamination, and change in backpressure through the system. Such changes are not contemplated in static schedules or functions determined under calibration conditions, and can therefore lead to inaccurate intake air dilution control and increased engine emissions.
It would therefore be desirable to provide for precision internal dilution control via variable valve timing control that does not require use of expensive or complex camshaft phaser position transducers and their associated complex and costly wiring and circuitry. It would further be desirable that such an internal dilution control account for any variation in dilution control parameter relationships over the life of the dilution control system, so that precise dilution control can be maintained despite any such variation.